EP0000702B1 - Process for forming a flow-resistant resist mask of radioation-sensitive material - Google Patents

Description

• The invention relates to a method for producing a flow-resistant mask from radiation-sensitive resist material.
• The formation of resist masks in the manufacture of integrated circuits using radiation-sensitive layers containing various organic polymers is known. Layers of a radiation sensitive material are applied to a support, exposed to light or other activating radiation such as electron beams or X-rays in an imagewise pattern and developed into a visible image by removing the more soluble portions of the layer in a developer solution. In order to improve the properties of the remaining parts of the resist layer, for example the adhesion to various substrates, their resistance to chemical and / or thermal degradation, the resist pattern is cured in a conventional manner at an elevated temperature. The thermoplastic polymer materials in the resist flow at temperatures above their glass transition temperature. This tendency to flow deforms the resist pattern, and in the high-resolution patterns, which are required, for example, for the production of integrated circuits, this deformation can lead to dimensional changes or even to the confluence of the fine lines. Such deformation can also take place if the resist image is heated during a process step carried out on the substrate, for example when using hot etching solutions, ion implantation or plasma etching. In order to overcome this disadvantage, free radical formers were incorporated into the photoresist, as described, for example, in German Offenlegungsschrift 2,518,480. With this method, resist masks with improved chemical and thermal resistance are obtained, but at the same time the radiation sensitivity of the resist materials is reduced, so that longer exposure times are required. U.S. Patent No. 3,920,483 describes a resist curing process for use prior to ion implantation in which the resist mask is subjected to high frequency gas plasma oxidation to reduce the thickness of the photoresist layer and limit the flow of the resist during ion implantation.
• The object of the invention is a method for producing a flow-resistant mask from radiation-sensitive resist material by curing, in which no special device is required for the curing process and in which the resist composition does not have to be modified before exposure.
• The object of the invention is achieved by a method of the type mentioned at the outset, which is characterized in that a curing agent of the o-quinonediazide type is applied to the mask, the coated mask is heated and then the excess curing agent is removed.
• Advantageous embodiments of the invention are laid down in the subclaims.
• The curing agents for use in the process according to the invention are water-soluble salts of o-quinonediazide sulfonic and carboxylic acids. Compounds of this type include o-quinonediazides from the benzene, naphthalene and phenanthrene series. Examples of the compounds have the formulas given below:

  

  

  in which R S0 ₃ X or COOX and X is for example Na +, K ⁺ , Ca ²⁺ , Ba2 +, Li ⁺ or NH ₄ ⁺ .
Examples of special connections are:
• The sodium salt of benzo) -2,1-diazo-oxide-4-sulfonic acid, the sodium salt of naphthalene-1,2-diazo-oxide-4-sulfonic acid, the potassium salt of naphthalene-1,2-diazo-oxide-4- carboxylic acid, the sodium salt of naphthalene-2,1-diazo-oxide-5-sulfonic acid, the sodium salt of 1,2-diazo-phenanthrol- (2) -x-sulfonic acid.
• The coating solutions of the hardeners are obtained in such concentrations represents that about 0.5 to 10 wt .-% of the curing agent, based on the total weight of the solution is present. Concentrations below 0.5% by weight may cause insufficient hardening to prevent the resist from flowing when heated. Also, no particular advantages are obtained using amounts greater than 10% by weight, and the presence of such amounts can cause crater or bubble formation in the resist mask pattern caused by gas evolution during heating.
• In practice, if resist layer areas without patterns are larger than 0.508x0.508 mm, the concentrations should be kept in a range of about 0.5 to 1.5% by weight in order to avoid crater formation in these continuous areas.
• The solvent system or the hardening agent is water-based, so that attack of the resist layer by the coating solution is avoided. It has been found that the resist layer is better wetted by the coating solution and, consequently, a more uniform coating of the curing agent is obtained when a mixture of water with alcohols with 2 to 4 carbon atoms, for example with ethanol, propanol or butanol, is used. The proportions of water and alcohol are chosen in such a way that sufficient solubility of the hardening agent is obtained and at the same time an attack by the solvent on the resist system is avoided. Instead of or in addition to the alcohols, surfactants can be added in amounts of about 0.01 to 1% by weight to improve wetting. Examples of useful surfactants are sodium lauryl sulfate, fluorocarbon based surfactants, sodium palmitate and polymethacrylic acid solutions.
• The curing agent is applied to the resist mask by conventional methods, for example by dipping, spraying or spinning, to obtain a continuous layer over the resist mask and the base. A layer ceiling of approximately 40 nm is required as a minimum in order to obtain a continuous coating on the surface and the side walls of the resist image. A layer thickness of approximately 50 to 100 nm is preferred. The desired layer thickness can be obtained in the spinning technique by varying the spinning speed. The spinning time is chosen so that a large part of the solvent is removed by evaporation.
• The resist compositions which can be cured by the process according to the invention can be either positive or negative resist materials. Negative resist materials are those that, when irradiated, crosslink and become less soluble. Examples of negative resist materials are sensitized polyvinyl cinnamate polymer compositions described, for example, in U.S. Patent 2,732,301 and sensitized, partially cyclized poly-cis-isoprene polymer compositions described, for example, in U.S. Patent 2,852,379. Examples of positive resist materials which degrade under the action of radiation and thereby become more soluble are sensitized novolak resin compositions which are described, for example, in US Pat. Nos. 3,046,118, 3,046,121, 3,201,239 and 3,666,743.
• The resist layers are applied to a base and exposed imagewise. The more soluble areas of the layer, which are the exposed areas in the case of a positive resist and the unexposed areas in the case of a negative resist, are then removed with a developer solution. The resulting resist mask pattern can then be treated in the process according to the invention by covering it with a layer of an o-quinonediazide curing agent. After coating, the resist mask is heated to a temperature which is sufficient to bring about a reaction of the o-quinonediazide with the resist layer to form a crosslinked outer layer, in particular on the side walls of the resist image, so that the lateral flow of the image is prevented. when the image is heated to temperatures above the glass transition temperatures of the polymer portion of the resist layer. Temperatures in a range from approximately 110 to 210 ° C. and an inert atmosphere, for example a nitrogen atmosphere, are used. Curing in an oven is preferred because fewer craters are formed in relatively large areas of the resist layer without a pattern compared to curing the resist layer on a hot plate. Hardening times of 10 to about 30 minutes are sufficient to complete the hardening.
• After the curing has been completed, the residue of the curing agent which remains on the base at the edges of the resist mask can be easily removed by rinsing with water. The resulting resist mask shows hardly any lateral flow of the images. The hardened resist image retains its dimensions during subsequent treatments of the exposed areas of the substrate, for example when etching with hot acids or with a reactive gas plasma, during ion implantation or a metal vapor deposition process in which the resist layer is heated to higher temperatures.
• The invention is explained in more detail using the following exemplary embodiments.
example 1
• A mask pattern of a positive photoresist was formed on the surface of a cleaned, metal-coated silicon semiconductor wafer: the resist composition consisted of a phenol formaldehyde novolak resin and the 2-diazo-1-oxo-naphthalene-5-sulfonic acid ester of dihydroxybenzophenone. The Resist was applied to the metal surface by spinning at 4000 revolutions per minute, cured to a dry layer thickness of approximately 2.2 μm for 20 minutes at approximately 85 ° C. and then imagewise exposed to actinic radiation and developed with an aqueous alkaline developer solution to expose the Remove areas. A 1% by weight solution of the sodium salt of 2-diazo-1-oxide-naphthalene-5-sulfonic acid in a mixture of water and isopropanol in a volume ratio was applied to the resist mask and the wafer by spinning at 3000 revolutions per minute within one minute 50:50 applied. The coating thickness was about 50 nm. The coated wafer was then cured in an oven under nitrogen for 20 minutes at a temperature of 210 ° C ± 5 ° C, and then the residue of the curing agent was removed by flushing with deionized water for 5 minutes . The curing agent prevented the resist images from flowing sideways, and the dimensional changes of the images were small (≤0.508 11m).
• The exposed areas of the metal layer were then etched by placing the wafer in a reactive gas plasma. The removal of the resist mask was uniform and the resist had a smooth surface after the metal etching process. This method enables the use of a thinner resist mask without attacking the parts of the metal layer lying under the mask during the plasma etching process. In contrast, resist layers on comparison wafers, which were only post-cured in the usual way or contained a peroxide additive in the composition, showed a rough surface with holes after the plasma etching.
• The use of the curing agent in concentrations in the range of 2.5 to 5% by weight according to Example 1 led to normal curing of the photoresist pattern, but large, 0.508 × 1.27 mm resist areas without a pattern showed craters which were evidently due to the bursting of enclosed gas bubbles caused during the curing process. When using a hardener concentration of 0.25% by weight, in which only small changes in the image size were caused, a considerable thinning of the narrow (about 2.54 11 m) resist lines was shown. The optimal hardener concentration should therefore be selected so that the flow is reduced to a minimum and at the same time crater formation in the special resist pattern to be treated is avoided. Concentrations of about 0.5 to 1.25% by weight gave good overall results for integrated circuit patterns.
Example 2
• A mask pattern of a positive photoresist with a thickness of about 1.5 11 m was formed on a silicon wafer, the surface of which was coated with a thermal oxide, as indicated in Example 1. The resist mask was coated using a solution of the sodium salt of 1-diazo-2-oxide-naphthalene-4-sulfonic acid in deionized water. Approximately 4 to 6 ml of the solution, which was filtered through a 0.5 J.Lm filter, was spun onto the resist images at a spinning speed of 3500 revolutions per minute within 1 minute. The wafer was then cured in an oven under a nitrogen atmosphere for 20 minutes at a temperature of 210 ° C ± 5 ° C. Photomicrographs of the resist image showed that flowing of the resist image was essentially avoided. In contrast, resist images on control wafers, which had not been subjected to the curing process according to the invention, showed a strong flow after curing at 210 ° C. Resist images which had been treated with peroxides such as Lupersol 101 (2,5-dimethyl-2,5-di (tert-butyl-peroxy) hexane or polymethacrylic acid solutions in deionized water and post-cured for 20 minutes at 210 ° C) also showed strong tiling and were deformed.
Example 3
• In order to demonstrate the application of the method according to the invention to negative-working resist materials, silicon semiconductor wafers, the surface of which was covered with a thermal oxide, were coated with a resist layer by spinning at 6000 revolutions per minute, which consisted of a partially cyclized poly-cis-isoprene polymer and 2. 6-bis (p-azido-benzylidene) -4-methylcyclohexane existed as a sensitizer. The layer was precured at 90 ° C for 20 minutes and then imagewise exposed to ultraviolet light for 1 minute. The exposed wafers were developed in a solvent at room temperature for 4 minutes to remove the unexposed areas and blown dry with nitrogen. The wafer was then split in half and one half was washed with an aqueous solution containing 4.3% by weight of the sodium salt of 2-diazo-1-oxide-naphthalene-5-sulfonic acid and 0.1% by weight. -% polymethacrylic acid coated. The other half was used for the control measurement. Both halves of the wafer were cured in a nitrogen atmosphere in an oven at 180 ° C for 30 minutes. The treated resist image showed a significantly improved dimensional stability of the image after curing. The resist image on the untreated half was severely melted. Similar results were obtained with another wafer, one half of which was treated and the other half of which was not treated, and both halves had been cured under nitrogen at 210 ° C for 30 minutes.
• A curing process for resist masks has been described, for which no special apparatus or additives to the resist layer are required before the resist mask is formed.

Claims (7)

1. Method of making a flow resistant mask of radiation-sensitive resist material on a substrate, characterized in that a hardening agent of the orthoquinone diazide type is applied to the mask, that the coated mask is heated and the excess hardening agent is then removed.

2. Method as claimed in claim 1, characterized in that water soluble salts of orthoquinone diazide sulfonic acid or carbon acid are applied as hardening agents.

3. Method as claimed in claims 1 and 2, characterized in that as hardening agents orthoquinone diazide compounds of the general formulas

are applied where R is S0₃X or COOX, and where X is selected from the group consisting of Na⁺, K+, Ca2+, Ba2+, Li⁺ or N H₄ ^+.

4. Method as claimed in claims 1 to 3, characterized in that the hardening agent is applied to the mask in the form of an aqueous or aqueous-alcoholic solution with a content of 0.5 to 10 percent by weight.

5. Method as claimed in anyone or several of claims 1 to 4, characterized in that the hardening agent is applied to the mask in a layer thickness of 50 to 100 nm.

6. Method as claimed in anyone or several of claims 1 to 5, characterized in that the coated mask is heated to a temperature between 110 and 210°C.

7. Method as claimed in anyone or several of claims 1 to 6, characterized in that the hardening agent is applied to a mask of a positive acting or a negative acting, radiation-sensitive resist material.